In multiple input multiple output (MIMO) communications systems, multiple antennas are used on both the base station and on the mobile device to exploit a phenomenon known as multipath propagation in order to achieve higher data rates. In general, MIMO communications systems simultaneously send and receive multiple data signals over each radio channel. The multipath propagation phenomenon is the result of environmental factors that influence the data signals as they travel between the base station and the mobile device, including, for example, ionospheric reflection and refraction, atmospheric ducting, reflection from terrestrial objects and reflection from bodies of water. Because of these factors, the data signals experience multipath interference that results in constructive interference, destructive interference, or fading, and phase shifting of the data signals. MIMO technology has been standardized in various wireless communications standards including Institute of Electrical and Electronics Engineers (IEEE) 802.11n, IEEE 802.11ac, HSPA+ (3G), WiMAX (4G) and Long Term Evolution (LTE) standards.
MIMO communications systems require testing. A typical MIMO test system for testing a mobile device under test (DUT) includes a base station emulator, a fading emulator, the mobile device under test (DUT), a personal computer (PC), some type of multi-probe configuration, and various electrical cables for interconnecting the components. In some MIMO test systems, the output ports of the fading emulator are connected to the antenna ports of the mobile DUT by electrical cables. This type of MIMO test system is known as a conducted MIMO test system. A disadvantage of this type of MIMO test system is that the mobile DUT has to be opened up, and in some cases broken, in order to access the antenna ports. The antenna connectors in the antenna ports are typically fragile and easily broken. Also, connecting the electrical cables to the antenna ports can be time consuming and can detrimentally impact the electrical performance of the mobile DUT. In addition, some mobile DUTs do not have antenna connectors in the antenna ports, and therefore cannot be tested using a conducted MIMO test system.
Another type of MIMO test system that is used to test mobile DUTs is a multi-probe anechoic chamber (MPAC) over-the-air (OTA) test system. In a typical MPAC OTA system, the mobile DUT is located inside of an anechoic chamber that includes a multi-antenna probe configuration. The output ports of the fading emulator are connected to the respective antenna probes of the chamber. This type of test system is currently the best available in terms of performance testing, but has drawbacks in terms of cost and space requirements. The anechoic chamber is expensive and typically requires at least ten square meters of floor space to employ. Also, the multi-antenna probe configuration typically utilizes many probe antennas and the fading emulator must have many channels to feed the probe antennas, which leads to high costs.
Another known MIMO test system uses a radiated two-stage methodology. The test set up is similar to that of the MPAC OTA set up. In the first stage, the mobile DUT is located inside of an anechoic chamber and its radiation pattern is measured. In the second stage, the mobile DUT is placed inside of another chamber that is equipped with N probe antennas, where N is a positive integer that is greater than or equal to two. During the second stage, the N×N transfer function for the N probe antennas and the N antennas of the mobile DUT is measured and its inverse multiplied by the channel model being emulated by the fading emulator. The performance of the test is about the same as that of the conducted MIMO test, but without the need for the cable connection to the antenna probes of the mobile DUT. The mobile DUT must be able to support a special mode that allows it to wirelessly communicate with the other test equipment of the system during testing, which is a disadvantage of this test system. Another disadvantage of this test system is that measuring the radiation pattern of the mobile DUT is a very time consuming process and requires the use of a costly anechoic chamber.
A need exists for a robust MIMO test system and method that eliminate the need to make wired connections to the antenna ports of the mobile DUT, that eliminate the need to use an anechoic chamber and the costs associated with it, that can be implemented in a relatively small area, and that can be set up and performed relatively quickly and easily.